The present invention relates generally to a silent chain used as a power drive chain in an automobile transmission drive unit, and more particularly to guide plates for a silent chain, the guide plates being arranged to sandwich toothed link plates of the chain in order to guide the chain as it travels around a sprocket.
FIG. 5 illustrates a conventional guide plate known from Japanese Patent Laid-open Publication No. HEI-8-35541. The guide plate is generally U-shaped and has a pair of apertures A, A formed in a pair of legs B, B, respectively, of the U-shaped guide plate. The legs B each have an outside flank C and an inside flank D. The inside flank D has a base portion D1 blended with an arcuate crotch D2. The crotch D2 extends upward to a position immediately below an upper end of each aperture A. By virtue of its arcuate or rounded shape, the crotch D2 is able to minimize the occurrence of fatigue cracks caused by stress concentration.
The aperture pitch, thickness and hardness of the guide plate are made smaller than those of the link plates, so that the guide plate has a lower yield strength than the link plates for ensuring that, after prestressed operation of the silent chain, the guide link pitch becomes substantially equal to the link plate pitch to thereby lessen the residual bending stress in the connecting pins.
However, for the aforesaid pitch equalization achieved by the prestressed operation, the guide plate is so designed as to undergo plastic deformation when subjected to a smaller yield load than the link plates. Accordingly, the durability of the guide plate is very low, and the fatigue strength of the chain as a whole is considerably reduced.
From a noise-reduction point of view, it is preferable that the guide plate have a spring characteristic or resiliency in the longitudinal direction of a chain. Due to its generally U-shaped configuration including a pair of legs B, B joined together by an round crotch D2 blended with the base portion of an inside flank D of each leg B at a position immediately below the upper end of an aperture A formed in each leg B, the guide plate operates as a rigid member when the link plates come into meshing engagement with a sprocket (not shown). The guide plate is, therefore, unable to exhibit the spring characteristic.
The guide plates, acting as rigid members, are unable to sufficiently absorb impacts occurring when the silent chain comes into meshing engagement with the sprocket. This allows the silent chain to oscillate like a vibrating string or cord with the result that collision sounds are produced upon repeated striking of the vibrating chain on the sprocket teeth. It is, therefore, hardly expected that the meshing noises resulting from the collision sounds can be reduced.
In addition, since a space formed between link teeth and sprocket teeth in mutual meshing engagement is completely closed from the opposite sides by the guide plates, compressive sounds are produced upon mutual meshing engagement between the link teeth and the sprocket teeth. The compressive sounds add to the meshing noises of the silent chain.
It is accordingly a general object of the present invention to overcome the drawbacks associated with the prior art.
A more specific object of the present invention is to provide a guide plate for silent chains which is capable of precluding stress concentration and absorbing impacts occurring when the silent chain comes into meshing engagement with a sprocket, thereby securing excellent durability and reduced meshing noises.
According to one aspect of the present invention, there is provided an improved guide plate for a silent chain having a chain pitch. The guide plate is substantially arch-shaped and has a pair of pin holes formed therein at a position adjacent to opposite free ends of the arch-shaped guide plate. The arch-shaped guide plate further has an arcuate inner peripheral surface and an arcuate outer peripheral surface both extending arcuately about a center located at the midpoint of a pitch line segment interconnecting the pair of pin holes. The arcuate inner peripheral surface has a radius of curvature which is 35 to 45% of the chain pitch, and the arcuate outer peripheral surface is blended with the opposite free ends of the arch-shaped guide plate.
According to another aspect of the present invention, there is provided a silent chain having a chain pitch and comprising a large number of link plates articulately connected together in an interleaved fashion by means of connector pins. Each of the link plates has a pair of pin holes through which connector pins of a pair of connector pins are inserted, respectively, and a pair of link teeth for meshing engagement with a sprocket and connected together by an arcuate crotch. Guide plates are disposed in pairs on opposite outer sides of the interleaved link plates for guiding travel of the silent chain around the sprocket. The guide plates are substantially arch-shaped, and bent toward the sprocket. Each of the guide plates has a pair of pin holes formed therein at a position adjacent to opposite free ends of the arch-shaped guide plate for fitting engagement with a pair of connector pins. The arch-shaped guide plate further has an arcuate inner peripheral surface and an arcuate outer peripheral surface both extending arcuately about a center located at the midpoint of a pitch line segment interconnecting the pair of pin holes. The arcuate inner peripheral surface has a radius of curvature which is 35 to 45% of the chain pitch, and the arcuate outer peripheral surface is blended with the opposite free ends of the arch-shaped guide plate.
It is preferable that the arcuate inner peripheral surface of each guide plate has a lowest part or bottom located closer to the sprocket than the top of the arcuate crotch of a mating one of the link plates, there being defined between the arcuate inner peripheral surface and the arcuate crotch a substantially almond-shaped space which opens at opposite ends in the widthwise direction of the silent chain.
The connector pin firmly engaged with a guide plate may be a rounded pin, or a rocker joint pin composed of a pair of pins of different lengths. When the rocker joint pin is used, the longer pin is firmly engaged with the guide plate.
The term xe2x80x9cpitchxe2x80x9d used herein is intended to refer a chain pitch, which is the distance between two adjacent pin holes. Especially when a round pin is used, the chain pitch is represented by the center distance between two adjacent pin holes. Alternatively, when a rocker joint pin is used, the chain pitch is represented by the distance between two adjacent long pins measured at respective faces held in rolling contact with corresponding faces of the shorter pins. This distance is substantially equal to the distance between a pair of pin holes formed in the guide plate.
The arcuate inner peripheral surface of the guide plate has a radius of curvature which is in the range of 35 to 45% of the chain pitch. By thus setting the range of the radius of curvature of the arcuate inner peripheral surface, the guide plate can possess an area around each pin hole which is sufficient to maintain the desired strength of the pin hole, and which is capable of displaying an appropriate spring characteristic or degree of resiliency without causing plastic deformation during the prestressed operation or usual power-transmitting operation of the silent chain. If the radius of curvature is below 35% of the chain pitch, stresses created in the guide plate during the prestressed operation or the usual power-transmitting operation cannot be limited within a range of elastic deformation of the guide plate. Conversely, if the radius of curvature exceeds 45% of the chain pitch, the guide plate is no longer able to maintain the necessary strength at portions or areas surrounding the pin holes.
The link plates used in combination with the guide plate may be of the so-called xe2x80x9cinside flank engagementxe2x80x9d type, in which the inside flank of one of teeth of each link plate comes first into sliding contact with the sprocket teeth at the beginning of meshing engagement, or of the so-called xe2x80x9coutside flank engagementxe2x80x9d type, in which the outside flank of one of teeth of each link plate comes first into sliding contact with the sprocket teeth at the beginning of meshing engagement. However, the inside flank engagement type link plats are preferable because the sliding sound of the link teeth can be reduced to thereby decrease the meshing noises of the silent chain as a whole.
Since the arcuate inner and outer peripheral surfaces of the arch-shaped guide plate extend about the center located at the midpoint of the pitch line segment interconnecting the pair of pin holes of the guide plate, and since the arcuate inner peripheral surface has a radius of curvature ranging from 35 to 45% of the chain pitch and the arcuate outer peripheral surface is blended with the opposite free ends of the arch-shaped guide plate, the guide plate functions as a rigid member during meshing engagement between the silent chain and the sprocket. However, since stresses created in the guide plate are distributed uniformly over the entire area of the arch-shaped guide plate, the guide plate is completely free from plastic deformation which would otherwise occur due to stress concentration. The arch-shaped guide plate can exhibit an appropriate spring action or resiliency which is effective to absorb impacts applied to the sprocket during meshing engagement between the silent chain and the sprocket.
In addition, since the bottom of the arcuate inner peripheral surface of the guide plate is located closer to the sprocket than the top of the arcuate crotch of a mating one of the link plates, there is a substantially almond-shaped space defined between the arcuate inner peripheral surface and the arcuate crotch. The almond-shaped space is open at opposite ends in the widthwise direction of the silent chain. Thus, a space defined between the link plate teeth and the sprocket teeth meshing with each other is not closed from opposite sides by the guide plates but allowed to communicate with the outside air. With this arrangement, mutual meshing engagement between the link plate teeth and the sprocket teeth does not produce compressive sounds, and an excess amount of lubricating coil trapped in the space between the link plate teeth and the sprocket teeth is permitted to flow smoothly toward opposite ends of the space which open in the widthwise direction of the silent chain.
The above and other objects, features and advantages of the present invention will become apparent to those versed in the art upon making reference to the following detailed description and accompanying sheets of drawings in which a preferred structural embodiment of the present invention is shown by way of illustrative example.